Slurry phase polymerisation of olefins is well known wherein an olefin monomer and optionally olefin comonomer are polymerised in the presence of a catalyst in a diluent in which the solid polymer product is suspended and transported.
Polymerisation is typically carried out at temperatures in the range 50-125° C. and at pressures in the range 1-100 bara. The catalyst used can be any catalyst typically used for olefin polymerisation such as chromium oxide, Ziegler-Natta or metallocene-type catalysts.
Typically, in the slurry polymerisation process of polyethylene for example, the slurry in the reactor will comprise the particulate polymer, the hydrocarbon diluent(s), (co) monomer(s), catalyst, chain terminators such as hydrogen and other reactor additives In particular the slurry comprises 20-80 weight percent (based on the total weight of the slurry) of particulate polymer and 80-20 weight percent (based on the total weight of the slurry) of suspending medium, where the suspending medium is the sum of all the fluid components in the reactor and will comprise the diluent, olefin monomer and any additives; the diluent can be an inert diluent or it can be a reactive diluent in particular a liquid olefin monomer; where the principal diluent is an inert diluent the olefin monomer will typically comprise 2-20, preferably 4-10 weight percent of the slurry.
Slurry polymerisation processes are typically carried out in continuous stirred tank reactors (CSTR) or loop reactors. Loop reactors are of a continuous tubular construction comprising at least two, for example four, vertical sections and at least two, for example four horizontal sections. The heat of polymerisation is typically removed using indirect exchange with a cooling medium, preferably water, in jackets surrounding at least part of the tubular loop reactor. The volume of each loop reactor of a multiple reactor system can vary but is typically in the range 10-200 m3, more typically 50-120 m3. The loop reactors employed in the present invention are of this generic type. The slurry is pumped around the relatively smooth path endless loop reaction system at fluid velocities sufficient to maintain the polymer in suspension in the slurry and to maintain acceptable cross-sectional concentration and solids loading gradients. Slurry is withdrawn from the polymerisation reactor containing the polymer together with the reagents and inert hydrocarbons, all of which mainly comprise inert diluent and unreacted monomer. The product slurry comprising polymer and diluent, and in most cases catalyst, olefin monomer and comonomer can be discharged intermittently or continuously, optionally using concentrating devices such as hydrocyclones or settling legs to minimise the quantity of fluids withdrawn with the polymer.
Control of the solids concentration in the reactor is important in order to avoid reactor blockage. Our own WO 2006/056761 discloses a polymerisation process in which the Froude number is maintained at or below 20 in order to operate at a relatively high solids concentration without employing high circulation velocities. The solids concentration in the reactor is typically 20-40 wt %.
Solids concentration has also been disclosed to influence the polymer properties in a process utilising two reactors in series. WO 02/29822 discloses a process in which the solids concentration of the first reactor is maintained at between 30 and 60 wt % for a Ziegler-Natta catalyst and between 35 and 60 wt % for a metallocene catalyst. This is claimed to result in improved product properties, as well as increased productivity. Both Ziegler-Natta catalysts (depending on the activator) and metallocene catalysts are known to have a “deactivating profile”, which means that polymerisation activity decreases with time.